The present invention is related to correction of pixel inaccuracy, and more particularly to correction for pixel inaccuracies in LED array printhead writers by thermal application.
The prior art has numerous references that disclose array writers, such as marking engines that are used in printers and copiers. Among these array writers are light emitting diode (LED) writers that are typically arranged as a single linear array or as multiple linear arrays. LED arrays will generally have some inaccuracies in pixel placement. There are various sources for the inaccuracy in pixel placement such as inherent manufacturing tolerance of the LED array or variability within the lens array that is used with the LED array, each of which can result in image placement distortion. The LED array will form an image on a receiver that moves in a direction referred to as the in-track direction and the inaccuracies in the in-track direction are referred to as bow. The in-track direction is perpendicular to the line in which linear arrays are formed, referred to herein as the cross-track direction. Inaccuracies in the cross track direction are referred to as length precision and are measured in terms of deviations from the nominal length of the LED array. The LED elements as arranged can exhibit inaccuracies in both the in-track and the cross-track directions.
Tandem writers are typically used for color printing, with each writer being responsible for a different color. Inaccuracy in pixel placement causes registration errors between the writers. These placement errors commonly result in color-to-color registration errors. Some of the pixel placement errors are caused by the mechanical placement error in the LED printhead assembly process; others are caused by lens variability and distortion on the images. The lens arrays as referred to herein are of the type, or similar to, SELFOC(copyright) (a trademark of Nippon Sheet Glass Company, LTD) lenses. Improvements have been made in the mechanical placement of LED arrays that are used in LED printhead substrates. The sorting of lens alleviates a major distortion problem, however, the sorting procedures are time consuming. Mechanical adjustments of the lens also provides a solution to distortion problems by compensating for image distortion using mechanical adjustment mechanisms such as using screws in the lens mount as discussed in U.S. Pat. No. 5,973,718; 1999 entitled xe2x80x9cMethod and Apparatus to Correct for Active Write Length and Bow Changes in LED print barsxe2x80x9d, issued to Charnitski, et al. (Charnitski). Charnitski provides a degree of correction, however, the amount of correct is limited. Additionally, matching a lens to a printhead has been found to provide a reduction in color-to-color registration errors in tandem writers. Typically, these prior art methods can reduce the bow error xcx9c30 to 40 xcexcm in an A3 size printhead, but are still a time consuming process. Electronic bow correction has been discussed in prior art disclosures such as U.S. Pat. No. 5,585,836 entitled xe2x80x9cElectrophotographic Image Recording Apparatus and Method with Correction for Bow in Placement of Recording Elementsxe2x80x9d, issued to Pham, et al. in 1996. U.S. patent application Ser. No. 09/870,305 (commonly assigned with the present invention) entitled xe2x80x9cCourse and Fine Electronic Bow Correction for a Writerxe2x80x9d in the name of O""Hara, et al., filed in May 2001, corrects pixel placement error in the in-track direction with a potential accuracy of better than 5-10 xcexcm. However electronic bow correction only takes into account misplacement of pixel elements in the in track direction, and provides no assistance for the errors that exist in the pixel placement in the cross-track direction. Similar amounts of error can be seen in the cross-track direction of the image as well. Without excessive sorting, that results in a reduction in yield and increases cost significantly, alternative methods are needed to further improve the process.
From the foregoing discussion, it should be readily apparent that there remains a need within the art for an apparatus and process that provides for correction in array writers in a cross track direction without requiring the sorting of pieces used to make up the array writers.
The present invention addresses the aforementioned shortcomings within the prior art and corrects the inaccuracies in pixel placement in the cross track direction by intentionally distorting pixel locations after the printhead has been integrated with the lens and the cross-track pixel position has been measured.
These and other objects of the invention are provided by the two embodiments of the invention. The first embodiments uses an LED printhead having a substrate (ceramic or other substrate) attached to a thermal electric cooler via the heatsink. The temperature on the substrate can be raised or lowered, to increase or decrease the linear dimensions of the printhead to compensate for the pixel placement error in the cross track direction. Radiometric data of the pixels (with the lens) is calibrated so that uniformity correction can be performed with the whole system. The second method assumes that there is a thermal electric cooling device attached to the SELFOC(copyright) Lens mount, so the lens can be stretched or contracted thermally to compensate for errors in the cross track direction of LED printhead from the determined nominal length. Radiometric data is taken and uniformity correction performed. The above method can be combined with the electronic bow correction of the in track direction to yield much better total pixel placement accuracy in both the cross track and in track directions and increase manufacturing yield with little sorting.